The invention relates to a method for providing analogs of human milk oligosaccharides (HMO), in particular oligosaccharides containing terminal sialic acid. The method comprises the steps of : a) providing a source of non-digestible galactooligosaccharides (GOS) containing at least two terminally bonded b-linked galactose residues； b) providing a sialic acid donor having (a2-3)-sialylated O-glycans； c) contacting said GOS with said sialic acid donor in the presence of an enzyme having trans-sialidase activity in an enzyme reaction mixture； and d) isolating from said enzyme reaction mixture a fraction comprising at least 20 percent by weight of disialylated galactooligosaccharides (di-Sia-GOS) based on the dry matter.
